Fuel cell with metal screen flow-field

ABSTRACT

A polymer electrolyte membrane (PEM) fuel cell is provided with electrodes supplied with a reactant on each side of a catalyzed membrane assembly (CMA). The fuel cell includes a metal mesh defining a rectangular flow-field pattern having an inlet at a first corner and an outlet at a second corner located on a diagonal from the first corner, wherein all flow paths from the inlet to the outlet through the square flow field pattern are equivalent to uniformly distribute the reactant over the CMA. In a preferred form of metal mesh, a square weave screen forms the flow-field pattern. In a particular characterization of the present invention, a bipolar plate electrically connects adjacent fuel cells, where the bipolar plate includes a thin metal foil having an anode side and a cathode side; a first metal mesh on the anode side of the thin metal foil; and a second metal mesh on the cathode side of the thin metal foil. In another characterization of the present invention, a cooling plate assembly cools adjacent fuel cells, where the cooling plate assembly includes an anode electrode and a cathode electrode formed of thin conducting foils; and a metal mesh flow field therebetween for distributing cooling water flow over the electrodes to remove heat generated by the fuel cells.

BACKGROUND OF THE INVENTION

This invention relates to electrochemical fuel cells, and, moreparticularly, to polymer electrolyte membrane fuel cells. This inventionwas made with government support under Contract No. W-7405-ENG-36awarded by the U.S. Department of Energy. The government has certainrights in the invention.

This application claims the benefit of U.S. provisional application Ser.No. 60/027,489, filed Sep. 27, 1996.

The fuel cell converts chemical energy to electrical power withvirtually no environmental emissions. A fuel cell differs from a batteryin that it derives its energy from supplied fuel, as opposed to energystored in the electrodes in the battery. Because a fuel cell is not tiedto a charge/discharge cycle, it can in principle maintain a specificpower output as long as fuel is continuously supplied.

One of the more commercially attractive types of fuel cells is thepolymer electrolyte membrane (PEM) fuel cell. A single PEM cell consistsof an anode and a cathode compartment separated by a thin, ionicallyconducting membrane. Hydrogen and oxygen (either pure or in air) aresupplied to the anode and cathode compartments, respectively. The PEMprevents hydrogen and oxygen from directly mixing, while allowing ionictransport to occur. At the anode, hydrogen is oxidized to produceprotons. These protons migrate across the membrane to the cathode andreact with oxygen to produce water. The overall electrochemical redox(reduction/oxidation) reaction is spontaneous, thus, energy is releasedas well. When several "unit" cells are combined in a stack, highervoltages and significant power outputs can be obtained.

The advantages offered by a PEM fuel cell (e.g. its low operatingtemperature and non-liquid, non-corrosive electrolyte) make itattractive as a potential energy source for transportation and forportable and stationary power applications. Fuel cells have beensuccessfully implemented in a number of utility, aerospace and militaryapplications, but the high cost of fuel cells compared to conventionalpower generation technologies has deterred their potentially widespreadcommercial adaptation.

The high costs are primarily due to the catalyzed membrane assembly(CMA) and the bipolar/flow-field plates. The CMA consists of thePEM/conductive backing structure which typically utilizes exotic andexpensive materials such as a platinum catalyst. Flow-field plates arecommonly graphite or specially coated metal plates that have beenmachined to contain channels through which gas flow is directed acrossthe plate. A bipolar plate has channels on each side to providereactants to the anode and cathode of adjacent cells in a stack and mayalso incorporate some form of cooling channels. Stack manufacturers havehistorically used high-platinum loaded CMAs and intricately machinedgraphite bipolar plates, which have made the cost of a fuel cell muchtoo expensive for most commercial applications.

An ideal bipolar plate would be a thin, light-weight, low-cost, durable,highly conductive, corrosion resistant structure that provides aneffective flow-field configuration. The conventional flow-field designconsists of a number of channels machined into a graphite or metal plateand is configured to provide relatively uniform reactant distributioncombined with effective water removal. Until recently, achieving aneffective flow-field design has come at the expense of tolerating thickand/or heavy bipolar plates with high material and machining costs.

The most commonly utilized bipolar plate material, graphite, isconductive and corrosion resistant, but it is expensive and not verydurable due to its brittleness. Titanium has been used to a lesserextent. Though it is extremely hard and can be treated (e.g., nitrided)to provide adequate conductivity and excellent corrosion resistance, itis prohibitively expensive, heavy, and difficult to machine.

A number of technologies are being considered to replace machinedbipolar plates in an effort to lower costs. Two of the more popularapproaches considered are (1) the use of composite materials, such asthe commercially available Kynar/graphite molded plates, and (2)relatively conventional flow-field/bipolar plate designs using massproduction metal fabrication techniques in contrast to the piecewisemachining currently being done. At present, these approaches haveyielded only small incremental changes in fuel cell costs.

In a departure from conventional flow-field designs and fabricationtechniques, U.S. Pat. No. 5,482,792 teaches the use of porouselectroconductive collectors to distribute reactants and reactionproducts and to distribute electrical current to the electrodes. In oneembodiment suitable collectors can be metal-wire fabrics or screens,wherein the wires form a series of coils, waves, or crimps, or otherundulating contour. The collectors are situated within a gasket framethrough which reactants are supplied to (and removed from) thecollectors by a series of channels. These channels span the width of thecollectors to attempt to evenly distribute reactants and reactionproducts. The gasket frame, which is made of a castable, elastomericmaterial (2 mm/0.079 in.) thick, seals against a metal frame, referredto in the patent as the bipolar plate. The bipolar plate separatesadjoining cells within a stack and is also used for cooling the cellswith which it is in contact. In the preferred configuration, the bipolarplate is made of aluminum and is necessarily thick (5 mm/0.197 in.) inorder to adequately withdraw heat generated by the cell. Bipolar platesmade of materials with lower thermal conductivities, such as stainlesssteel, are thinner (3 mm/0.118 in.), but have a more complicated designin order to accommodate extra channels for forced air cooling.

In addition to lowering or eliminating machining costs, another primaryobjective in bipolar plate design is to minimize thickness. While thismay be of secondary importance for stationary power applications, theweight and size of the fuel cell stack has substantial implications intransportation applications. Minimizing bipolar plate thickness lowersstack weight, volume, and cost of materials, with a concomitant increasein the fuel cell power density. Stacks of individual unit fuel cellsbased on graphite hardware typically have low cell pitches (e.g., 1.6cells/cm, 4 cells/inch) because the bipolar plates must be sufficientlythick to avoid cracking. Even though the stacks disclosed in U.S. Pat.No. 5,482,792 use metal components, a low pitch is still obtainedbecause of the overall stack configuration.

Accordingly, it is an object of the present invention to provide asimpler and more effective reactant and cooling flow distributionconfiguration than provided by known bipolar plates.

Another object of the present invention is to provide uniform reactantdistribution over a fuel cell membrane without machined bipolar plates.

Yet another object of the present invention is to provide thin, compact,and relatively light-weight cell "cartridges" that are mini-stackscontaining two or more individual fuel cells, wherein adjacentcartridges are separated by a cooling plate when combined to form largerstacks.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

SUMMARY OF THE INVENTION

To achieve the foregoing and other objects, and in accordance with thepurposes of the present invention, as embodied and broadly describedherein, the apparatus of this invention may comprise a polymerelectrolyte membrane (PEM) fuel cell having electrodes supplied with areactant on each side of a membrane electrolyte assembly (CMA). The fuelcell includes a metal mesh defining a flow-field pattern having an inletconfined to a first corner and an outlet confined to a second cornerlocated on a diagonal from the first corner, wherein all flow paths fromthe inlet to the outlet through the square flow field pattern areequivalent to uniformly distribute the reactant over the CMA.

In a particular characterization of the present invention, a bipolarplate electrically connects adjacent fuel cells, where the bipolar plateincludes a thin metal foil having an anode side and a cathode side; afirst metal mesh on the anode side of the thin metal foil; and a secondmetal mesh on the cathode side of the thin metal foil. In anothercharacterization of the present invention, a cooling plate assemblycools adjacent fuel cells, where the cooling plate assembly includes ananode electrode and a cathode electrode formed of thin conducting foils;and a metal mesh flow field therebetween for distributing cooling waterflow over the electrodes to remove heat generated by the fuel cells.

The fuel cells of the present invention may also form a fuel cellcartridge. At least two fuel cells are connected with adjacent ones ofthe fuel cells separated by electrically conductive, thin foil bipolarplates. A first thin foil bipolar plate forms a cathode at a first endof the connected fuel cells and a second thin foil bipolar plate formsan anode at a second end of the connected fuel cells. First and secondwire mesh flow fields contact the first and second thin foil bipolarplates, respectively, for removing heat generated by the connected fuelcells.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate the embodiments of the present inventionand, together with the description, serve to explain the principles ofthe invention. In the drawings:

FIG. 1 is a cross-sectional schematic of a fuel cell with a metal screenflow-field in accordance with one embodiment of the present invention.

FIG. 2 is a plan view of a metal screen flow-field shown in FIG. 1.

FIG. 3 is a schematic illustration of flow distribution over a metalscreen flow-field

FIG. 4 is an exploded view of a fuel cell according to one embodiment ofthe present invention.

FIG. 5 is an exploded schematic of an exemplary catalyzed membraneassembly used in the fuel cell shown in FIG. 1.

FIG. 6 is an exploded view of a fuel cell cartridge according to oneaspect of the present invention, where the cartridge is formed from twounit cells.

FIG. 7 is an exploded view of an assembly of cartridges, where adjacentcartridges are separated by removable cooling plates.

FIG. 8 graphically depicts performance characteristics of a fuel cellusing a stainless steel screen flow-field and metal foil separatorplates.

FIG. 9 graphically depicts fuel cell resistance as a function of celloperating hours for the cell configuration shown in FIG. 1.

FIG. 10 graphically depicts fuel cell operating parameters after 24hours and 800 hours of operation.

FIG. 11 graphically depicts fuel cell resistance over 2000 hours ofoperation.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, fuel cell thickness is reducedby using thin metal or carbon foils in place of conventional bipolarplates. The fuel cell flow-fields are based on wire mesh configurations,which may be simple diagonal path-equivalent patterns formed of variousmetals including stainless steels for uniform water and reactantdistribution over a fuel cell electrolyte assembly (CMA) or bipolarplate surface. FIG. 1 is a cross-sectional schematic of one embodimentof a fuel cell 10 that incorporates woven wire-mesh screens asflow-fields for the reactants and for cooling: anode (hydrogen)flow-field 12, cathode (oxygen or air) flow-field 14, and, optionally,coolant flow-field 16. In one aspect of the invention, bipolar foil 20is formed by anode flow field 12 of cell 10, cathode flow-field 13 of anadjacent fuel cell, and conducting foil 32. CMA 18 is sandwiched betweenflow-fields 12 and 14, and includes catalyzed membrane 22 sandwichedbetween backings 24 and 26, further discussed below. Flow-fields 12, 14,16 are sandwiched between thin metal foils 32, 34, 36, respectively, tocreate a thin fuel cell configuration for use in a fuel cell stack,i.e., a series-connected array of fuel cells, to reduce the volume of afuel cell stack and increase the overall power density.

Major advantages of using simple flow-fields, such as woven wirescreens, include the elimination of expensive raw materials costs andmachining and/or other special fabrication costs. Many types of screensare readily available in a variety of thicknesses and mesh sizes. Anadditional benefit of the screens is that they are relativelylight-weight in comparison to thick graphite or solid metal plates.Other advantages with metal screens are 1) the screens are not brittleand very thin unit cells may be possible, 2) sheet metal and screenfabrication and die-cutting are "commodity" processes compared to thespecialized composite molding processes, and 3) metal screens may permitmore effective stack designs in that tolerances can be more precise andunit cells may possibly be thinner than with the molded composites.

Referring now to FIG. 2, a diagonal path-equivalent flow-field 42 inthis invention is configured such that the reactant flow introduction islimited to one corner, e.g., corner 44, and the effluent discharge islimited to the opposing corner, e.g., corner 46. In one embodiment usinga wire screen, wire screen 48 is oriented such that the wires of theweave are parallel to the edges of the screen, which is sealed aroundthe periphery with gasket 52 except for the opposing manifolded corners.

In the over-and-under weave of a simple square weave mesh wire screenshown in FIG. 3, flow between the small box regions defined by theinterwoven wires can only be between elements with common sides. The twowires stacked on one another that define a corner of an element preventsflow between diagonal elements in the interweave sections. Thus, if oneconsiders the simple schematic in FIG. 3, the flow can only be in the(x) or (y) directions through the weaves of the screen.

Two possible flow-paths are shown in FIG. 3. Path (a) proceeds straightin the (-y) direction and then straight in the (+x) to the exit whileclosely passing by the distant lower-left corner. Path (b) proceedsalong the diagonal, but because of the x-y flow preference, the flowfollows a staircase-like path. Once the total length of all theindividual segments in the staircase are summed, the lengths of paths a)and b) are identical. Therefore, the flow rate along each path should beequivalent. In practice, the flow is suprisingly uniformly distributedthroughout the entire flow-field and there are no dead spots in the farcorners as may be intuitively expected.

The flow configuration shown in FIG. 3 was confirmed by the use of awire screen square flow-field mock-up in a transparent plastic housingto observe the reactant flow distribution and water accumulation in asimulated cathode flow-field. Air was introduced at 3 atm pressure at aflowrate equivalent to 1 A/cm² at 50% oxygen utilization and humidifiedabove the saturation point within the cell to replicate two-phase flowconditions. The water vapor would fog the cool surface of the housing.Condensed water droplets passing through the flow-field would wet thewall and clear the surface until it had a chance to fog again. It wasthus possible to observe the condensed water streamlines. The patternsformed by the streamlines demonstrated that the flow was uniformlydistributed over the entire flow field, even to the distant corners.Water removal was effective and there were minimal and only temporaryaccumulations within the flow-field.

As contemplated by the present invention, the shape of the flow-fieldsis not limited to a square. Rectangles based on square or rectangularmeshes also provide equivalent flow-paths with corner-to-corner flow. Inaddition, the corner-to-corner elements can be a building block tolarger arrays. For example, the active area of a cell could be nelements wide. Two inlets could be positioned at 1n and 3n along the topedge, and the three outlets could be placed at 0n, 2n, and 4n along thebottom. Each nth element is then supplied with corner-to-corner flow.The manifold penetrations for the flow-field for the opposing electrode(or the flow-field on the backside of the plate) would then occupy theoff-corners to the first flow-field. A two-dimensional extension of thisconcept would result in arrays in both directions with manifoldpenetrations in the active area that each supply (or receive) flow tofour corner-to-corner elements oriented around it.

An advantage of using corner-to-corner flow in an internally manifoldedfuel cell stack 60, shown in FIG. 4, is in the simplicity andcompactness of the manifold to flow-field transition. Since the twodifferent sets of opposing corners, e.g., 84 and 86 in gasket 64 and 88and 92 in gasket 66, can be used for the anode and cathode reactantflows, a simple channel connecting a manifold penetration to each corneris all that is needed, as is shown in FIG. 4. In one embodiment, slots84/86 and 88/92 are cut in gaskets 64 and 66, respectively to define theflow routes. In another embodiment, the gaskets are replaced by thin,rigid plastic frames coated with adhesive for sealing. Largedistribution channels, as typically used with numerous small, parallelchannels or with "porous" flow-fields are not required, which simplifiesthe configuration and allows more of the footprint to be activeelectrode area.

Another, more subtle, advantage lies in the nature of the flow dynamics.With corner-to-corner flow across wire-screens 68, 72, 74, and 76, therelative space velocity of the reactants is much greater at the entranceand exit. When operating the cathode on air (or the anode on a dilutehydrogen stream), this acts to increase the uniformity of the currentdistribution over conductive backings 112, 114 (FIG. 5) compared to amore conventional set-up with a constant space velocity in all areas(neglecting reactant consumption).

In order to maintain the integrity of the membrane/cooling plateassembly 62 during assembly and sealing, it is another feature of thepresent invention to form the CMA shown in FIG. 5 as a rigid frame withconductive, gas diffusion backings 112, 114 pressed against opposedsurfaces of membrane 102. In an exemplary embodiment, the periphery ofmembrane 102 around the active area is sandwiched between two 0.254cm/0.010 in. thick SS sheets 104, 106 cut into the appropriateconfiguration. Industrial strength adhesive (0.051 mm/0.002 in. thick)is first applied to SS sheets 104, 106 and the components are bondedtogether under pressure. Membrane 102 is then trapped in a structurethat is easily handled and does not change dimensions. This simplifiesassembly and part reproducibility. The picture frame assembly can bepretested for the integrity of membrane 102 (e.g. no gas cross-over).The stiffness of the structure prevents membrane 102 from falling intothe channels cut into gaskets 64, 66 (FIG. 4) to connect manifolds 94A,D, E, H (FIG. 4) to the flow-field (e.g., channels 84/88, 92/86, in FIG.4) which would cause leakage of the reactants in the gap thus formedopposite of the channel.

In another embodiment, a thin, rigid plastic frame combines the gasketand stainless steel picture frame (e.g., frame 106 and gasket 66). Theplastic frame, which is made of polysulfone, is coated on both sideswith a high bond adhesive (0.051 mm/0.002 in. thick), and resemblesframe 106 in that it traps the CMA in a rigid structure. However,because a separate gasket 66 is no longer required, the plastic framemust provide reactant access from the manifold hole to the flow-field.Channels corresponding to, e.g., slots 88, 92 in gasket 66, are formedas grooves from the opposing corner manifold holes to the open activearea in the gasket side of the combined frame. The gasket side of theframe contains the flow-field hardware and is adhered directly to theconducting metal foil, e.g., foil 82. The other side of the frame issmooth (no grooves) and is adhered to membrane 102 (FIG. 5).

In an exemplary embodiment, membrane 102 (FIG. 5), which is situatedbetween the anode 68, 74 and cathode 72, 76 screens, consists of a 100cm² active area catalyzed Nafion™ membrane (Nafion 112 from DuPont) andtwo E-TEK carbon cloth backings 112, 114 (each about 0.457 mm 0.018 in.thick, 100 cm²). To give the CMA physical stability, membrane 102 isenclosed within a thin metal "picture frame" (FIG. 5). A high-bondadhesive is applied to one side of each of frames 104, 106 (stainlesssteel foil, 0.254 mm/0.010 in. thick each). These frames are die-cut toform penetrations for the manifolds (FIG. 4, 338 94A-H) and the windowfor the active area of membrane 102. The ionomeric membrane issandwiched between the two adhesive-coated sides of the frames and theassembly is pressed together. Membrane 102 is trimmed from the manifoldholes using standard cork borers. Carbon cloth backings 112, 114 arethen pre-pressed to the membrane at ambient temperature andapproximately 1.2 atm of pressure. The finished product of CMA 62 (andbackings) encased within a rigid frame assures proper alignment of CMA62 within the cell and enhances sealing.

Referring again to FIG. 4, unit cell 60 is built around the pictureframe CMA 62. In a preferred embodiment, the wire-screen flow-fields oneither side each consist of two metal screens 68, 74 and 72, 76 (each10×10 cm) that sit within a gasket "frame" 64, 66, respectively, that isapplied to a metal foil 78, 82, respectively. Relatively coarse screens74, 76 (24×24 mesh, 0.356 mm/0.014" diameter wire) serve as the main gasflow-fields and are placed directly against foils 78, 82. A fine screen68, 72 (60×60 mesh, 0.191 mm/0.0075" diameter wire), is laid on top ofeach coarse screen 74, 76 flow-field. Fine screens 68, 72 serve as aphysical barrier between coarse screens 74, 76 and carbon cloth backings112, 114 (FIG. 5) of CMA 62, which is located between the anode andcathode flow-fields. The tight weave of fine screens 68, 72 acts to keepcarbon cloth backings 112, 114 of CMA 62 from "falling" into the openweave of coarse screens 74, 76, but still allows reactant and watervapor to reach the CMA.

In practice, however, a fine screen can collect water over time. Thesquare openings formed by the tight weave act as hydrophilic "cages,"trapping water and decreasing reactant access to the CMA. This problemhas been overcome by providing a hydrophobic coating on the fine screensprior to use to prevent water from collecting in the openings. In oneembodiment, the fine screens were pretreated with a mixture of carbonblack and Teflon.

If a relatively soft material is used for conductive backings 112, 114without fine screens 68, 72 in place, the backing becomes embedded inthe coarse screen, resulting in relatively high pressure drops and cellresistance result. An exemplary effect of the fine screens is shown inTable A.

                  TABLE A                                                         ______________________________________                                                      Pressure Drop                                                                            Cell Resistance                                      Fine Screen   psi        Ω-cm.sup.2                                     ______________________________________                                        Without       6          >0.5                                                 With          2          0.15                                                 ______________________________________                                    

Additionally, thin, flattened expanded metal mesh and perforated platescan and have been utilized in place of the fine screen. Perforatedplates in particular appear to provide an advantage to fine screens interms of water management within the cell. By design, a perforated platecan have larger "openings" and "islands" that more nearly replicate theconfiguration obtained with conventional channeled flow-fields and stillprovide adequate reactant access to the CMA, protect the backing fromblocking the coarse screen, and yet be very thin. The coarse screencontinues to serve as the flow field for water distribution. Because athin perforated plate with large openings does not tend to trap water(such as the fine weave screen does), it is not necessary tohydrophobize the structure. Perforated plates have been made of 316stainless steel (SS). with a thickness of only 0.127 mm/0.005 in. andwith penetrations of 1.016 mm/0.040 inches diameter on a 1.324 mm/0.060in. stagger.

It is a particular feature of this invention to use thin metal foilseparators 78, 82 (0.254 mm/0.010 in. thick in this embodiment) adjacentflow-field screens 68/74 and 72/76, respectively to be used in a bipolarconfiguration. As shown in FIG. 1, in a stack of unit cells 10, the setof screens 12 on one side of foil 32 would be the anode flow-field forone cell, whereas screens 13 on the other side of foil 32 would be thecathode flow-field for the neighboring cell. The assembly of screens 12and 13 with foil 32 forms a novel bipolar configuration with screenflow-fields. The metal screens provide some structural support andeliminate the need for machining of the metal separator plates so thatthin foils may be used. As used herein, the term "thin" means metalfoils having a preferred thickness less than about 0.508 mm/0.20 in. inorder to minimize the composite thickness of a unit cell 60 (FIG. 4).

As shown in FIG. 1, cooling may be provided by sandwiching two of themetal foil separators 34, 36 around a cooling flow-field 16, and theanode and cathode flow-fields then sandwich the foil separators. Eitherof the remaining sets of manifold penetrations, e.g., manifoldpenetrations 94B/G or 94C/F (FIG. 4) can be used for the coolant. Aswith the reactant flows, a channel is formed in the gaskets adhered toplates 34 and 36 (FIG. 1) to connect the manifold to the coolantflow-field. This flow-field may be a wire screen or a serpentine channelcut into a compressible, electronically conductive material such asgraphite gaskets (e.g. Grafoil from Union Carbide). FIG. 1 shows coolingflow field 16 adjacent cathode 34, but the cooling flow field could alsobe adjacent anode 32.

Channels 84, 86 and 88, 92 (FIG. 4) for the corner-to-corner flow arecut directly into gasket frames 64 and 66, respectively. The gaskets aremade of 1.524 mm/0.060 in. thick fabric-reinforced silicone tape. Thesilicone is a closed-cell foam, which can be easily compressed toprovide a gas-tight seal, yet not require excessive force to accomplishan effective seal. This supplies substantial leeway in matching the sealthickness with the thickness of the compressed cell. The compression ofthe cell may be controlled by introducing rigid "tabs", e.g., tabs 65and 67, of the appropriate thickness located at the beveled outercorners of the gasket frame to control the compression of the cell. Assuch, the gasket can be compressed to no less than the thickness of thetabs, which assures uniform compression across each cell. This isimportant in a stack, because each cell must be pressed to the samedegree to assure equal distribution of reactant flow and uniformity ofcell performances. In the embodiment in which a rigid plastic framereplaces the gasket/stainless steel picture frame, tabs are unnecessarybecause the frame itself serves to control compression of the cellcontents.

The unit cell 60 depicted in FIG. 4 is approximately 3 mm/0.118 in.thick, providing for a thin, compact design in which the active areacomprises about 60% of the total area encompassed by the metal foils andframes. Because the individual cell components are adhered togetherbetween metal foils, the resulting structure is a one-piece unit that isreferred to herein as a "cartridge." A cartridge can contain one orseveral cells that share separator foils as well as cooling plates.However, there are advantages to retaining the cooling plate as aseparate structure.

In an exemplary structure shown in FIG. 6, cartridges are formed of twocells 60. Larger stacks are assembled by combining cartridges togetherwith removable cooling plates that separate the units, as further shownin FIG. 7.

An exemplary cartridge 120 is formed from two unit cells 60 that areconnected in series at bipolar plates 78/82, which are thin metal foils.More than two unit cells may be connected, as shown in FIG. 6, ifcooling considerations permit. It will be understood that the componentsof unit cell 60 correspond to like-numbered components in FIG. 4 and arenot discussed separately with respect to FIG. 6.

In a particular feature of the present invention, FIG. 7 depicts coolingplates 126 between adjacent cartridges 120. Cooling plates 126 includewire screens 124 that are held in a frame assembly 132, that may alsoserve to electrically connect adjacent cartridges 120 in series, ifdesired. Cooling flow is input/output through manifold openings 126/128and is evenly distributed over wire screen 124, as discussed above. Wirescreens 124 may be simply woven mesh screens, but may be any suitablescreen that provides fluid communication through the cooling volumebetween adjacent fuel cells.

In a test configuration, a cartridge is fixtured between gold-platedcopper current collector plates, which in turn are placed between andelectrically isolated from aluminum end-plates that incorporate fittingsto supply the internal manifolds. Tie-bolts connecting the two endplatesprovide mechanical compression of the unit cells.

In summary, the use of flow connections to opposing corners of a squaremesh wire screen or other flow-field material provides a configurationsuch that the effective lengths of all possible paths (thatincrementally advance towards the outlet) are equivalent. Not only doessuch a scheme appear to be effective, but it greatly simplifies theconfiguration of the manifolding around the periphery in that it allowsthe fuel and oxidant reactant manifold penetrations to occupy the twosets of opposing corners and not overlap or interfere with one another.

The picture frame packaging of the CMA allows effective sealing whenused with the wire screen flow-fields. A picture frame configuration hasadvantages even with conventional hardware because of the improvedsealing surfaces and dimensional stability of the structure from thepoints of view of testing, handling, and assembly.

Additionally, the overall packaging of individual fuel cells intocartridges enables the quick and easy assembly of stacks and also makesit possible to remove and replace problem cells within a stack withoutcompromising the remaining cells. Because the cartridges are relativelythin, high power density stacks are possible that are relatively small,compact, and light-weight.

A polarization curve for a 100 cm² single cell assembled as describedabove is shown in FIG. 8 operating on humidified, pressurized H₂ /air(3/3 atm, 50% air utilization at 1 A/cm²). The CMA used in the pictureframe consisted of a Nafion 112 membrane with low platinum loadingcatalyst layers (0.14 mg Pt/cm² /electrode). The high frequency cellresistance was less than 0.15 Ω cm², which is high compared to smallcells, but not unreasonable considering the numerous components andinterfaces involved and the size of the active area. In any case, thecell provided about 0.5 W/cm² at 0.62 V (50% LHV voltage efficiency).

Cells with 304 SS wire screens and foils demonstrated loss of componentconductivity with cell operation. The use of 316 SS instead of the 304improves the stability of the metal structures. Thus, a cell wasassembled using 316 SS components that were not surface treated beyondwashing with soap and water to remove surface grime and then swiped withan organic solvent such as isopropanol or acetone to remove any lasttraces of grease or soap, or the like.

One such cell was operated for 2,000 h at a constant voltage of 0.5 V.Operating temperatures ranged from 70°-80° C. for the cell itself,90°-110° C. for the hydrogen gas humidifier, and 70°-80° C. for airhumidification. Both the hydrogen and air gas streams were pressurizedat 3 atm, and flow rates were maintained at approximately 2 and 4L/min., respectively. These flows roughly correspond to 2× thestoichiometric amount of flow required to operate the cell at 1 A/cm².The cell was not internally cooled, instead it was air-cooled by anexternal fan.

The long term performance of the cell was monitored in a number of ways:by measuring voltage drops across the cell components, measuring highfrequency resistance (HFR), and by periodically obtaining polarizationcurves. The stability of the metal hardware was quantified using voltageleads connected to the various metal components. With the cell set tooperate at a fixed current, the voltage drops were measured between thevarious components, which allows the calculation of the contribution ofthe metal components to the cell resistance.

The 2,000 hour results are shown in FIG. 9. The anode and cathode metalcomponent contributions in this figure consist of the resistances of thecomponents and interfaces between the fine screens (positioned againstthe CMAs) and their respective current collector plates. Thus thecurrent traverses 1) the SS fine screen, 2) the SS flow-field screen, 3)an SS separator plate, and 4) the gold-coated current collector plate,which is a total of four components and three interfaces. Also shown inthis figure is the total contribution of the metal hardware, calculatedfrom the measured voltage difference between the fine wire sense leadsand the current collector plates at a fixed current. The curves in FIG.9 demonstrate that the contribution of the metal hardware is relativelystable. The exception to this conclusion occurred at about 800 hourswhen the tie-bolts holding the cell together were re-tightened, whichbrought the components together more forcefully and substantiallyimproved the conductivity across the interfaces. The improvement in cellperformance realized upon the tightening of the tie-bolts isdemonstrated in FIG. 10. From the improvement in the low-current densityregion of the curves, it is evident that the performance was due torealizing more effective electrode performance rather than to thedecrease in component resistance alone.

If the stainless steel was corroding or passivating, the voltagemeasurements across the screens would tend to increase over time. Thiswas not observed in FIG. 9, which suggests either that corrosion wasminimal or that the corrosion products were soluble. In the latter case,dissolution of the metal hardware would liberate polyvalent ions thatcould conceivably enter into the polymer electrolyte membrane and tie upactive sites, adversely affecting the protonic conductivity of theionomer. To some extent, this can be monitored by high frequencyresistance (HFR), as it is a diagnostic tool in determining thecondition of the CMA. As observed in FIG. 11, the final HFR did notincrease over the initial value, although it did fluctuate with time.However, the HFR is susceptible to the hydration state of the membrane.Thus it is difficult to quantify any possible decrease in theconductivity due to the inclusion of metal ions, but the figure suggeststhat this is not occurring to any appreciable extent, if at all.

In any event, the 316 SS hardware was clearly superior to the 304 SS interms of long term stability. After disassembly of the 2,000 hour 316 SScell, no visible corrosion of the metal components was apparent. Perhapseven better results may be obtained with 316L SS because of its lowercarbon content and corresponding corrosion susceptibility. These resultsare very promising because the use of a relatively low-cost metal alloywithout the need for any special surface treatments or coatings providesa very low-cost fuel cell stack technology.

The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed, andobviously many modifications and variations are possible in light of theabove teaching. The embodiments were chosen and described in order tobest explain the principles of the invention and its practicalapplication to thereby enable others skilled in the art to best utilizethe invention in various embodiments and with various modifications asare suited to the particular use contemplated. It is intended that thescope of the invention be defined by the claims appended hereto.

What is claimed is:
 1. A polymer electrolyte membrane (PEM) fuel cellhaving electrodes supplied with a reactant on each side of a catalyzedmembrane assembly (CMA), said fuel cell comprising;a reactantdistribution metal mesh defining a rectangular flow-field pattern havingan inlet confined to a first corner and an outlet confined to a secondcorner located on a diagonal from said first corner, wherein all flowpaths from said inlet to said outlet through said rectangular flow fieldpattern have equivalent lengths to uniformly distribute said reactantover said CMA.
 2. A PEM fuel cell according to claim 1, wherein saidreactant distribution metal mesh is a square weave screen.
 3. A PEM fuelcell according to claim 2, further including a support structure forsupporting said reactant distribution metal mesh on said CMA.
 4. A PEMfuel cell according to claim 1, wherein each one of said electrodes is abipolar plate comprising an anode electrode and a cathode electrodeformed of thin conducting foils and said metal mesh flow fieldtherebetween for distributing cooling water flow over said electrodes toremove heat generated by said fuel cell.
 5. A PEM fuel cell according toclaim 4, further including a support structure for supporting saidreactant distribution metal mesh on said CMA.
 6. A PEM fuel cellaccording to claim 1, wherein said CMA is a rigid structure including arigid frame structure bonded to and supporting said membrane withconductive backings pressed into said membrane.
 7. A PEM fuel cellaccording to claim 6, wherein said metal mesh is a square weave screen.8. A PEM fuel cell according to claim 6, further including a supportstructure for supporting said reactant distribution metal mesh on saidCMA.